Question

the third period contains only 8 element even through the electron capacity of the third shell is 18

Answer 1

the period 3 of the periodic table has those elements whose atoms have three shells occupied by electrons
according to Aufbau's principle that can be only 8 elements that satisfy this. This is because the S orbital of a fourth shell (4S) is at lower energy level than the orbital of the D orbital of the third Shell (3D) with the s and p orbitals of the third Shell are filled, the next element potassium has its outermost electrons in the S orbital
therefore, question..

Answer 2

the period 3 of the periodic table has those elements whose atoms have three shells occupied by electrons

according to Aufbau's principle that can be only 8 elements that satisfy this. This is because the S orbital of a fourth shell (4S) is at lower energy level than the orbital of the D orbital of the third Shell (3D) with the s and p orbitals of the third Shell are filled, the next element potassium has its outermost electrons in the S orbital

therefore, question